This invention relates to a control system of an alternating current motor wherein the motor is energized by a separate excitation type frequency converter including serially connected rectifier and inverter.
Although the invention is applicable to the control of an alternating current motor energized by a constant current type inverter of the indirect frequency converter type or a cycloconverter of the indirect frequency converter type, in the following the invention will be described as applied to the control of an induction motor which is energized by a well known constant current type inverter as shown in FIG. 1, wherein alternating current supplied to an input terminal 11 is rectified by a rectifier 12, the direct current is smoothed by a DC reactor 13 and then converted into alternating current by an inverter 14 for energizing an induction motor 15. As diagrammatically shown, the rectifier and inverter comprise semiconductor switching elements such as thyristors or power transistors which are connected in a manner well known in the art. In the current type inverter, the output voltage is sinusoidal and the output current is rectanqular, whereas in the voltage type inverter the output voltage is rectangular but the output current is substantially sinusoidal. The former type is suitable for large current and can regenerate the power of the alternating current motor by changing the phase control angle .alpha. of the rectifier from 0.degree.-90.degree. to 90.degree.-180.degree..
The control system shown in FIG. 1 comprises a reference setter 21 which produces a speed reference signal e.sub.1, an input limiting circuit 22 which is constructed to modify the reference signal e.sub.1 in accordance with the current flowing through the frequency converter for limiting the current, a voltage/current controller 23 which is constructed to compensate for the voltage drop of the converter circuit caused by the primary current of the motor 15 and to control the primary current, and a phase controller 24 which in response to the output signal e.sub.4 of the voltage/current controller 23 changes the phase control angle .alpha. of the rectifier 12 from 0.degree.-90.degree. to 90.degree.-180.degree., and vice versa, depending upon whether the rectifier 12 operates as a rectifier or an inverter at the time of regeneration. The inverter 14 is provided with a frequency controller 25 which is constructed to control the output frequency of the inverter. Since these circuit elements are well known in the art, description of the detail thereof is believed unnecessary.
It is assumed now that the AC voltage supplied to the induction motor 15 is controlled by a closed loop in which a speed reference signal e.sub.1 and an output voltage feedback signal e.sub.2 are compared and that the frequency of the alternating current is controlled by an open loop utilizing a digital control signal obtained by passing the speed reference signal e.sub.1 through an analogue-digital converter. When such control system is used, the torque of the induction motor 15 is determined by such factors as the output voltage, the current and frequency of the inverter output and the slip frequency or power factor of the induction motor, but with such control system as the power factor (cos .theta.) of the load is not controlled, unstable phenomena occur under specific load conditions wherein the output voltage and current of the inverter oscillate at low frequencies.
Such unstable phenomena are caused by various factors but it has been considered that such phenomena are caused mainly by a high output impedance of the frequency converter and by the fact that the phase of the output current is fixed. More particularly, when the induction motor 15 is subjected to such external disturbances as load variations, the phase of the induced electromotive force and hence the induced magnetic flux of the induction motor vary and such phase variation and the output of the frequency converter undergo mutual interference thereby causing low frequency oscillations. As above described, since such low frequency oscillations are caused by the induction motor it is impossible to suppress them by the output voltage control loop of the frequency converter so that the low frequency oscillations persist.